Automotive vehicles having independent suspensions are generally equipped with stabilizer bars to reduce inclination or roll of the vehicle bodies during vehicle maneuvers. The stabilizer bar is usually connected between the suspension arms of the vehicle wheels. When the left and right wheels are in similar positions with respect to the suspensions, the stabilizer bar does not twist so that the suspensions are mutually independent. When one of the left wheel and right wheel passes over a bump on the road surface, or when the vehicle turns and thus the left wheel and the right wheel assume considerably different positions with respect to the suspensions, the stabilizer bar is twisted. This twisting motion induces a torsional resilient force for affecting the handling and ride performance characteristics of the vehicle wheels.
It is desirable that the torsional resilient force of the stabilizer bars can be adjusted in accordance with ride and handling conditions of the vehicle. Specifically, it is desirable to reduce the torsional rigidity during straight travel of the vehicle and to increase the torsional rigidity during turning of the vehicle. The reduced torsional rigidity enhances the ride and handling characteristics of the vehicle wheels while the increased torsional rigidity enhances the handling and ride characteristics of the vehicle.
Certain vehicle active tilt control systems include front and rear stabilizer bars which are adjustable by front and rear hydraulic actuators placed in lieu of the stabilizer bar linkages. The actuators are movable in first and second opposing directions for adjusting vehicle body active roll moment to compensate for vehicle roll.
Some prior art vehicle active control systems include numerous accumulators in the system which act as gas springs for accumulating energy in order to respond to system demand and fill-in flow when needed. Such systems are typically very expensive because they employ numerous valves and some include two accumulators per corner, which increases cost.
The quality of motion of the actuators is very important for a comfortable ride. The evaluation of the motion quality is based on actuator motion quality criteria. Mainly, for any motion control system and for an active tilt control system as well, there are apparent advantages to having smooth actuator motion. The smooth motion is motion without discontinuities in actuator trajectories. "Monotonic" motion is defined as a change in the velocity vector angle only in one direction during a single stroke.
For example, when the velocity vector angle only decreases during the actuator motion, the motion can be considered high quality motion. Motion where the velocity vector angle is decreasing and increasing during a single directional movement is considered low quality. These two situations are illustrated in FIGS. 1 and 2. As shown in FIG. 1, the actuator continues to move toward the end stop without reversing velocity vector gradient, while in FIG. 2, the actuator actually changes direction of velocity vector gradient, or jerks, when moving toward an end stop.
In most motion control systems where linear or rotary actuators are implemented, it is very desirable to have decreasing velocity toward the end stop of the actuators. Velocity close to zero at the end stop would be the ideal case.
An active tilt control system is not exempted from these rules. If the actuator's velocity is not decreased toward the end stop, it is likely that the transient roll angle will overshoot. Even if the overshoot is not present, roll motion will not be comfortable for the passengers. In addition to the ride quality, there are other reasons for the decrease of the actuator speed at the end of travel. One reason is that the violent change of acceleration when the actuator slams into the end stop. The acceleration in this case is a scaled version of the supply pressure. If the pressure transient within the actuator's chamber is not smooth, and has break points or discontinuities or high frequency oscillations, the actuator components will suffer damage, which results in reduced life expectancy for such components. In other words, large acceleration changes result in large forces which may damage components and result in an uncomfortable ride for vehicle occupants.
Discontinuities in actuator motion are considered break points that represent sudden change in the actuator velocity angle. This change in the active tilt control system case means that actuators actually speed up at the end stop. The front and rear actuator strokes illustrated in FIGS. 3a and 3b are not synchronized, which means when one actuator bottoms out, fluid enters the chamber of the moving actuator, which accelerates the actuator toward the bottom out position. This is a highly undesirable situation.
Different automotive suppliers have proposed using numerous hydraulic accumulators to achieve functional and comfortable ride quality. Similarly, the use of spool valves has been introduced to distribute the flow between the actuators to smooth out the ride. However, such configurations are highly expensive.
It is therefore desirable to provide an improved vehicle active tilt control system in which actuator movement is cushioned near the end stops for improving quality of the system and reducing component wear without a substantial cost penalty.